Abstract

The Baltic Sea population of the common eider (Somateria mollissima) has declined dramatically during the last two decades. Recently, widespread episodic thiamine (vitamin B1) deficiency has been demonstrated in feral birds and suggested to contribute significantly to declining populations. Here we show that the decline of the common eider population in the Baltic Sea is paralleled by high mortality of the pulli a few days after hatch, owing to thiamine deficiency and probably also thereby associated abnormal behaviour resulting in high gull predation. An experiment with artificially incubated common eider eggs collected in the field revealed that thiamine treatment of pulli had a therapeutic effect on the thiamine status of the brain and prevented death. The mortality was 53% in untreated specimens, whereas it was only 7% in thiamine treated specimens. Inability to dive was also linked to brain damage typical for thiamine deficiency. Our results demonstrate how thiamine deficiency causes a range of symptoms in the common eider pulli, as well as massive die-offs a few days after hatch, which probably are the major explanation of the recent dramatic population declines.

Analytical results of experimental common eider (Somateria mollissima) pulli from the County of Blekinge 2013. (a) Mortality was low or absent in the two thiamine treatment groups but high in the control group, i.e. death was prevented by thiamine treatment. (b) Minimal neuropil vacuolation in the brain was associated with inability to dive. (c–d) Thiamine treatment had a therapeutic effect on both brain SumT and proportion brain TDP (the Kruskal-Wallis test, followed by the exact Wilcoxon-Mann-Whitney test as a post-hoc test). (c) Brain SumT. (d) Proportion brain TDP. (e–f) In both liver and brain, there was a negative relationship (ordinary least squares regression) between proportion TDP and T concentration, indicative of various degree of thiamine deficiency. (e) Liver. (f) Brain. ● Box plots drawn according to Tamhane & Dunlop. C = control.